Reentrant diaphragm with central closure member



June 20, 1950 A. E. GERSCH 2,512,323

REENTRANT DIAPHRAGM WITH CENTRAL CLOSURE MEMBER Filed Sept. 9, 1946 2 Sheets-Sheet 1 June 20, 1950 A. E. GERSCH 2,512,323

REENTRANT DIAPHRAGM WITH CENTRAL CLOSURE MEMBER Filed Sept. 9, 1946 2 Sheets-Sheet 2 0002 Q 2 Q Q d 000/ U) E Q Q U 2 d 3 Q a 00/ LE 7 men-1701" b W W9 as S'JNAU O/XZ' sway g0) sansssa 01W? 05 I Patented June 20, 1950 REENTRANT DIAPHRAGM WITH CENTRAL CLOSURE MEMBER Adolph E. Gersch, Boonton, N. J., assignor to Radio Frequency Laboratories, Inc., Boonton, N. J a corporation of New Jersey Application September 9, 1946, Serial No. 695,645

3 Claims. 1

This invention relates to sound reproducing devices and more particularly to a novel loud speaker of the dual-cone type having improved frequency response characteristics.

Although the range of audible vibrations extends from 30 to 16,000 cycles per second, loud speakers, in general, have a response range from approximately 60 to 7000 cycles per second. It is known that a single, vibratory diaphragm is incapable of satisfactory response over the limited range of 60 to 7000 cycles as the low frequencies necessitate the use of a relatively large, flexible diaphragm whereas the relatively high frequencies necessitate the use of a small, rigid diaphragm. Attempts to design a single diaphragm to fulfill these two requirements have not been successful because the mass of the large, low frequency section loads the high frequency section to the point where the overall response of the speaker falls oil objectionably at the i.

sive to the lower range of frequencies and a small inner cone section reversely disposed with respect to the large cone section and which is responsive to the higher range of frequencies. This procedure results in a loud speaker of shallow construction which feature is most desirable in view of the reduced space factor required in small radio receiving sets, phonographs, etc. However, while dual-cone speakers of this type are responsive over a greater range of frequencies than are single cone speakers, their response still falls 01f sharply at frequencies lying below the desired maximum value. Many attempts have been made to flatten out the response of such speakers by careful design of the two cone ections but experience has shown that a factor of correction applied. to a given frequency range introduces new errors in other ranges. In short, manipulation of the design constants of the diaphragm section results in a shifting of the response characteristic along the frequency axis but does not aiford basic correction in an overall sense.

A loud speaker made in accordance with this invention possesses a definite and pronounced improvement in frequency response while retaining the advantage of shallow construction.

An object of this invention is the provision of a loud speaker having a sound output level, within the recognized acceptable output-band, that 2 extends over a greater range of frequencies than has been possible heretofore.

An object of this invention is the provision of a loud speaker comprising a motor, a dual-cone diaphragm actuated by the motor and means modifying the frequency response characteristic of one of the cones whereby the sound output of the speaker is increased at the higher frequencies.

An object of this invention is the provision of a loud speaker comprising two cone sections forming the vibratory diaphragm, a single motor actuating both cone sections and a member of non-porous material closing the inside of one of the cone sections.

An object of this invention is the provision of a loud speaker comprising a motor, a vibratory diaphragm actuated by the motor, said diaphragm having an inturned apex forming an inner cone section, and a non-porous, relatively stiff member closing the inside of the inner cone section substantially at the base thereof.

An object of this inventoin is the provision of a loud speakerin which the voice coil operates in a closed air-chamber.

These and other objects and advantages will be apparent from the following description when taken with the accompanying drawings which illustrate several embodiments of the invention. The drawings are for purposes of illustration and are not to be construed as defining the scope or limits of the invention, reference being had for this purpose to the appended claims.

In the drawings wherein like characters refer to like parts in the several views:

Figure 1 is a central, cross-sectional view of a loud speaker made in accordance with this invention;

Figure 2 is an isometric view of the loud speaker shown in Figure 1;

Figures 3 and 4 are fragmentary, cross-sectional views showing other forms of the member forming a closure for the inner cone section; and

Figure 5 is a set of curves illustrating speaker response with respect to frequency.

Referring now to Figures 1 and 2, the numeral I0 identifies a metal frame that serves as a support for the speaker as a whole. The frame can be made of a single punching in accordance with established practise and it includes a solid, central section H which serves as a base for the permanent magnet l2 and the soft-iron yoke l3. Operative within the magnetic flux-gap established by the permanent magnet is a voice coil sidered as acceptable.

I4 wound upon a tube I5 of light-weight material. A conventional spider I6 made of a highlyresilient material is cemented to the yoke and serves both to center the voice coil it! within the flux-gap and an additional purpose described in more detail hereinbelow.

The vibratory diaphragm comprises a single sheet of fibre or other suitable material formed in the shape of a large frustro-conical section-J1 that is secured to the frame I by a clamping ring I8, and a small frustro-conical section la that is secured to the tube I by cement, substantially as shown. To facilitate unrestricted vibrationof the diaphragm a section of high resiliency .isprovided by the corrugations as is well known in the art.

In operation the voice ooilvibrates in response to electrical oscillations and the vibrations there of are transmitted to the inner cone section I9 through the medium of the tube I5. The small diameter cone section I9 possesses substantial radial stiffness and is, therefore, responsive to frequencies in the higher ranges. On the other hand, low frequency vibrations are transmitted by the small cone section I9 to the large'cone section I! which'fundamentally is better adapted .to

'thispurpose. .As is well known, at low frequencies (up to about 400 cycles) the large cone section I! reciprocates as a unit piston eificiently compressing relatively large air masses. At the highfrequencies (3,000-5,000 cycles) the small inner cone section I9 reciprocates as a unit piston. In the frequency range between 400 and 3,000 cycles the large cone section is no longer capable of operation-as a piston, insteadthe section operates with a ripple motion along itssurface thereby holding the output level up until the period of the small cone section is attained. .At frequencies beyond 5,000 cycles the piston movement of the small cone section dies away and the motion likewise becomesa ripple motionalong thesurface. The output response of the loud speaker asfa whole drops off rather rapidly whenthe small cone section is no longer capable of operation as apiston. A perfect speaker would be one having an ab solutely linear, or constant,.output response over the entire range of operating frequencies. Any increase or decrease in sound pressure at specific frequencies represents a deviation from normal but from a practical consideration certain deviations are not as objectional as others. Specifically, negative deviations from normal at the higher frequencies are particularly objectionable as full bodied tones are rendered pleasing principally by reason of the'highfrequency harmonic contentand removal of these harmonics results in a'fiat,'lifeless tone devote of natural quality.

An actual response curve takenon a dual-cone speaker of representative, present day quality is shown by the solid line curve A in Figure 5. Industry standards specify that an output level, or sound pressure, of90 to 110 decibels (db) is con- It is to be noted that the speaker output falls within the acceptable range between 80 and 4,200 cycles per second but it suffers a sharp decrease in the regions of 1,500 .and

' 2,500 cycles.

ceptable output band over a wider range of frequencies. I accomplish this by closing completely "the'large diameter base of the inner cone section I9 with a cap 2| that is made of non-porous, relatively stifi material. The addition 'or the cap '2! to the same speaker from which the curve A, Figure 5, was obtained, results in the curve B. A

comparison of the two curves indicates a substantially similar response characteristic at the 5 lower frequencies, but the objectionable low points at 1,500 and 2,500 cycles have been eliminated so that the output of my improved speaker falls within the acceptable db band without spurious deviations. Further, the range of acceptable response has been extended to approximately 6,000 cycles per second. Actual operating tests have proven that the improved quality of my speaker is astonishingly apparent to individuals skilled in this art and also is apparent immediately to the average human ear.

I believe the fundamental theory underlying the operation'oi my improved loud speaker can be :explained in part, by a consideration of the mechanics relating to the operation of the small cone I9. The cap 2| completes the closure of an air chamber defined'by the cone section I9, tube I5, yoke I3 and the base I I. The escape path for the air so confined lies along the magnetic fluxgap and throughthe'material Of which the'spider I6 is made. While the spider may be porous to a certain degree, it serves nevertheless as a pressure vent and effectively retards the exchange of air between the closed chamber and the sur rounding atmosphere. Assume now that the cone section I'Qstarts tomove from its at rest position in a direction such that the air within the chamber tends to be compressed. Because of the relatively large volume of the entrapped air and the resiliency of the spider IS, the air offers only a negligible retarding force to oppose initial movement of the cone. However, as the motion of the cone approaches a maximum in accordance with the intensity of the imparted vibration the retarding effect of the air undergoing compression becomes more pronounced. By proper design of the air chamber in terms of volume arid the resiliency of "the spider, the retarding effect of the air can be made significant only at the point of maximum deflection of the cone. In this manner the motion of the cone can be cushioned to prevent overswing with the consequent result that the lag between the actual motion of the cone and the vibrations of the voice coil is materially reduced.

Similar considerations apply to motion of the cone in the reverse direction in which case the increase in the volumetric content of the air chamber brings into action the rarefication rather than the compression of the entrapped air.

From the above considerations it is apparent that the construction of a diaphragm in accordance with this invention increases the natural period of the inner cone thereby increasing its com- 30 pliance at the higher frequencies.

It is now also apparent that cap 2! may be given a variety of shapes to meet specific response requirements and to effectuate improved response quality in a speaker having a given size 55 motor, diaphragm, etc. Figures 3 and 4 illustrate other, representative forms of the cap being identified by the numerals 22 and 23 respectively.

Having now described my invention certain variations and modifications will now be apparent 70 t those skilled in the art. Such variations and modifications are intended to fall within the scope and spirit of the invention as defined by the claims.

I claim:

1. A vibratory member for a loud speaker comprising a single diaphragm in the form of a truncated cone having the center section turned inward to thereby form an outer frustro-conical section and an inner, reversely disposed frustroconical section; a relatively rigid, non-porous closure member secured across and closing the large diameter base of the inner frustro-conical section said closure member lying in a plane substantially normal to the axis of the diaphragm; and a tubular member secured at one end to the small diameter base of the said inner frustroconical section and coaxial therewith.

2. The invention as recited in claim 1, wherein the closure member is in the form of a flat plate disposed normal to the axis of the inner frustroconical section.

3. The invention as recited in claim 1, wherein the closure member is in the form of a plate having a convex central portion.

ADOLPH E. GERSCH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,793,889 Wood Feb. 24, 1931 1,925,659 Giles Sept. 5, 1933 2,034,882 Scribner Mar. 24, 1936 2,186,576 Cornwell Jan. 9, 1940 2,197,649 Neuschotz Apr. 16, 1940 2,295,483 Knowles Sept. 8, 1942 FOREIGN PATENTS Number Country Date 319,455 Great Britain Sept. 26, 1929 796,655 France Apr. 11, 1936 435,670 Great Britain Sept. 25, 1935 

